There's a short answer to your question and a longer fuller answer. The short answer is that a common standard for road cranksets is 130mm BCD (bolt circle diameter). The smallest inner chainring you can use with 130 BCD is a 39 (as Kibbee has said, technically you can get a 38 tooth 130 BCD chainring although that doesn't leave much metal between the lower edge of the teeth and the mounting holes). Front derailleurs and rear derailleurs can handle a change in chainring size of perhaps 13 or 14 teeth smoothly and reliably without a problem; larger differences than that can be handled but usually not quite as crisply or reliably. That's why larger front chainrings are often 52 or 53.
Some bikes with triple chainrings can use a much smaller BCD. The old TA Cyclotourist crankset favored by randonneurs could use a small chainring of 24 teeth. However, when the BCD is very small the chainrings must be stronger to prevent warping since there is so much distance from the teeth to the mounting holes at the crank.
Track bikes use a 144 BCD because track riders often use large chainrings (and, of course, only a single ring in a fixed gear set-up).
A relatively recent standard is 110 BCD cranksets that permit a small chainring of 34 teeth. This is often paired with a larger chainring of 50. That is a difference of 16 teeth, which is right on the edge of smooth and reliable shifting with current chainring and derailleur designs. As Rider_X has noted, this recent standard was enabled by new designs that allow for stiff chainrings.
So the BCD determines the smallest chainring that can be used, while the derailleurs put a limit on the largest increase in teeth from small to large chainring that can be handled smoothly and reliably.
Note that two different chainring-cog combinations may have the same gear ratio (for example, a 48-24 and a 36-18 both would have a gear ratio of 2.0). If they have the same gear ratio, then at the same bike speed, your cadence would be the same. Since the bike speed is the same, the power reaching the ground must be the same. So gear ratio is the same, speed is the same, cadence is the same, power is the same -- but chain tension will be different. At the same cadence, say 60 rpm for simplicity, the chainring will make one complete revolution per second. That means that a 48 tooth chainring will move the chain 48 links (or 24 inches) per second, while a 36 tooth chainring will move the chain 36 teeth (or 18 inches) per second. Since the same power must travel from the crank to the rear wheel, the force transmitted through the chain must be higher for the slower moving chain. As an aside, this happens to have been the way that one bicycle power meter worked (the Polar chain speed and tension power meter): it had a sensor to measure chain speed, and another to measure chain tension.